1. Technical Field
The present invention relates to an optical isolator with improved stability and, more particularly, to the utilization of a Faraday rotator configured to provide improved stability of the optical isolation with respect to variations in parameters such as temperature or wavelength.
2. Description of the Prior Art
Reflections in optical systems often generate noise and optical feedback which may degrade the performance of various components, particularly semiconductor lasers. Therefore, the ability to optically isolate lasers and other sensitive components from these reflections is critical to the performance of the system. The Faraday effect in magneto-optic material provides a unique non-reciprocal device capable of performing the isolation function. In general, a conventional optical isolator comprises a 45.degree. Faraday rotator encased in a bias magnet and disposed between a pair of polarization selective means (e.g., linear polarizers, birefringent plates, or birefringent wedges) oriented at an angle of 45.degree. to each other. Signals passing through the isolator in the transmitting, forward direction will be essentially unaffected by the polarization selective means and rotator. However, return reflections will be rotated such that the signal will be essentially blocked from the propagating back into the signal source (e.g., laser).
The ability of an isolator to block reflected signals is frequently expressed in terms of the "extinction ratio" (ER), where ER (in dB) is defined as -101 og (I.sub.R /I.sub.0, I.sub.0 being defined as the incoming reflected intensity parallel to the output polarizer and I.sub.R as the outgoing reflected intensity from the input polarizer towards the source (e.g., laser). Assuming ideal polarization selective elements (at a 45.degree. angle in relative orientation), the extinction ratio ER can be expressed as -101 og (sin.sup.2 .THETA..sub.T), where .THETA..sub.T is defined as the departure from 45.degree. in Faraday rotation. For example, a conventional magneto-optic material such as yttrium iron garnet (YIG) exhibits a temperature variation in Faraday rotation of approximately 0.04.degree./.degree.C. at a wavelength of 1300 nm (for a conventional 45.degree. Faraday rotator). A material such as a commercially available bismuth-substituted rare earth iron garnet is known to exhibit an even higher temperature-dependent change in Faraday rotation of approximately 0.06.degree.-0.07.degree./.degree.C. These changes in rotation as a function of temperature thus change the rotation imparted on a signal passing therethrough and, hence, the degree of isolation provided by the Faraday rotator.
U.S. Pat. No. 4,756,687 issued to N. Watanabe et al. on Jul. 12, 1988 discloses an exemplary arrangement addressing the temperature stability problem. In particular, Watanabe et al. dislose a cascaded isolator arrangement comprising a pair of isolators, the first including a 45.degree. Faraday rotator tuned to a wavelength .lambda..sub.1 slightly less than the system wavelength .lambda..sub.0, and the second including a 45.degree. Faraday rotator tuned to a wavelength .lambda..sub.2 slightly greater that .lambda..sub.0. The temperature-induced variation in rotation of the first Faraday rotator is thus substantially cancelled by the rotation deviation of the second isolator stage. Although the Watanabe et al. arrangement does provide improvement in terms of temperature stability, the resulting structure is at least twice the size of a conventional isolator and exhibits increased signal loss (as a result of additional components and tuning of the rotators away from the nominal system wavelength).
Another problem with conventional optical isolators is their stability as a function of transmitted signal wavelength. In particular, garnet will exhibit a change in rotation as a function of wavelength. U.S. Pat. No. 4,712,880 issued to M. Shirasaki on Dec. 15, 1987 discloses an isolator arrangement including polarization compensation means for providing an isolator design which is less sensitive to drift in the system wavelength. The Shirasaki arrangement comprises a first birefringent wedge plate; a polarization rotation compensator composed of a combination of a half-wave plate and a quarter-wave plate; a 45.degree. Faraday rotator; and a second birefringent wedge. The polarization rotation compensator may be designed to provide nearly linear isolation over a wavelength range of, for example, 1.3 .mu.m to 1.5 .mu.m. As with the above Watanabe et al. arrangement, however, the Shirasaki isolator requires a number of additional components which adds to the overall coast and complexity of the resulting design.
Therefore, a need remains in the prior art for an optical isolator capable of providing temperature and wavelength stability without unduly increasing the cost, size or complexity of the resulting arrangement.